MindFeather asked in viewtopic.php?f=20&t=1928 about curves. Right now this isn't very well supported by Grid Cartographer but I thought I'd at least have a stab at a work-around and provide a download to help out as best I can.
I made this using the following custom tiles (drag&drop these images into your map, choose mono/tintable and use the X/Y flip options in the Tool Opts. panel to form a circle)
or Download Map
Share image exports, map files and custom tiles here.